.\" $OpenBSD: EVP_PKEY_derive.3,v 1.11 2024/07/21 08:25:33 tb Exp $
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.Dd $Mdocdate: July 21 2024 $
.Dt EVP_PKEY_DERIVE 3
.Os
.Sh NAME
.Nm EVP_PKEY_derive_init ,
.Nm EVP_PKEY_derive_set_peer ,
.Nm EVP_PKEY_CTX_get0_peerkey ,
.Nm EVP_PKEY_derive
.Nd derive public key algorithm shared secret
.Sh SYNOPSIS
.In openssl/evp.h
.Ft int
.Fo EVP_PKEY_derive_init
.Fa "EVP_PKEY_CTX *ctx"
.Fc
.Ft int
.Fo EVP_PKEY_derive_set_peer
.Fa "EVP_PKEY_CTX *ctx"
.Fa "EVP_PKEY *peerkey"
.Fc
.Ft EVP_PKEY *
.Fo EVP_PKEY_CTX_get0_peerkey
.Fa "EVP_PKEY_CTX *ctx"
.Fc
.Ft int
.Fo EVP_PKEY_derive
.Fa "EVP_PKEY_CTX *ctx"
.Fa "unsigned char *key"
.Fa "size_t *keylen"
.Fc
.Sh DESCRIPTION
.Fn EVP_PKEY_derive_init
initializes the public key algorithm context
.Fa ctx
for shared secret derivation using the
.Vt EVP_PKEY
object already stored in
.Fa ctx .
The library provides built-in support for keys with an
.Xr EVP_PKEY_base_id 3
of
.Dv EVP_PKEY_DH ,
.Dv EVP_PKEY_EC ,
.Dv EVP_PKEY_HKDF ,
and
.Dv EVP_PKEY_X25519 .
.Pp
After the call to
.Fn EVP_PKEY_derive_init ,
algorithm specific control operations can optionally be performed
to set any appropriate parameters for the operation.
.Pp
.Fn EVP_PKEY_derive_set_peer
configures the
.Fa ctx ,
which already needs to be initialized with
.Fn EVP_PKEY_derive_init ,
.Xr EVP_PKEY_encrypt_init 3 ,
or
.Xr EVP_PKEY_decrypt_init 3 ,
to use the
.Fa peerkey ,
which is normally a public key.
In case of success, the reference count of the
.Fa peerkey
is incremented by one.
Consequently, the caller needs to call
.Xr EVP_PKEY_free 3
on the
.Fa peerkey
when the caller no longer needs it, even if it is still in use by
.Fa ctx .
.Pp
.Fn EVP_PKEY_derive
derives a shared secret using
.Fa ctx .
If
.Fa key
is
.Dv NULL ,
then the maximum size of the output buffer is written to the
.Fa keylen
parameter.
If
.Fa key
is not
.Dv NULL
then before the call the
.Fa keylen
parameter should contain the length of the
.Fa key
buffer.
If the call is successful, the shared secret is written to
.Fa key
and the amount of data written to
.Fa keylen .
.Pp
The function
.Fn EVP_PKEY_derive
can be called more than once on the same context if several operations
are performed using the same parameters.
.Sh RETURN VALUES
.Fn EVP_PKEY_derive_init ,
.Fn EVP_PKEY_derive_set_peer ,
and
.Fn EVP_PKEY_derive
return 1 for success and 0 or a negative value for failure.
In particular, a return value of \-2 indicates the operation is not
supported by the public key algorithm.
.Pp
For
.Fn EVP_PKEY_derive_set_peer ,
a return value of \-1 can for example occur if
.Fa ctx
is not properly initialized, does not contain an
.Vt EVP_PKEY
that can be retrieved with
.Xr EVP_PKEY_CTX_get0_pkey 3 ,
the
.Xr EVP_PKEY_id 3
of both keys mismatch, or
.Xr EVP_PKEY_cmp_parameters 3
reports mismatching key parameters.
.Pp
.Fn EVP_PKEY_derive
fails with a return value of \-1 for example if
.Fa ctx
has not been successfully initialized with
.Fn EVP_PKEY_derive_init .
.Pp
.Fn EVP_PKEY_CTX_get0_peerkey
returns an internal pointer to the
.Fa peerkey
used by
.Fa ctx
without incrementing its reference count.
.Sh EXAMPLES
Derive shared secret (for example DH or EC keys):
.Bd -literal -offset indent
#include <openssl/evp.h>
#include <openssl/rsa.h>

EVP_PKEY_CTX *ctx;
unsigned char *skey;
size_t skeylen;
EVP_PKEY *pkey, *peerkey;

/* Assumes that pkey and peerkey have already been set up. */
ctx = EVP_PKEY_CTX_new(pkey, NULL);
if (!ctx)
	/* Error occurred */
if (EVP_PKEY_derive_init(ctx) <= 0)
	/* Error */
if (EVP_PKEY_derive_set_peer(ctx, peerkey) <= 0)
	/* Error */

/* Determine buffer length */
if (EVP_PKEY_derive(ctx, NULL, &skeylen) <= 0)
	/* Error */

skey = malloc(skeylen);

if (!skey)
	/* malloc failure */

if (EVP_PKEY_derive(ctx, skey, &skeylen) <= 0)
	/* Error */

/* Shared secret is skey bytes written to buffer skey */
.Ed
.Sh SEE ALSO
.Xr EVP_PKEY_CTX_new 3 ,
.Xr EVP_PKEY_decrypt 3 ,
.Xr EVP_PKEY_encrypt 3 ,
.Xr EVP_PKEY_meth_set_derive 3 ,
.Xr EVP_PKEY_sign 3 ,
.Xr EVP_PKEY_verify 3 ,
.Xr EVP_PKEY_verify_recover 3 ,
.Xr X25519 3
.Sh HISTORY
.Fn EVP_PKEY_derive_init ,
.Fn EVP_PKEY_derive_set_peer ,
.Fn EVP_PKEY_CTX_get0_peerkey ,
and
.Fn EVP_PKEY_derive
first appeared in OpenSSL 1.0.0 and have been available since
.Ox 4.9 .
